Applicant's U.S. Pat. No. 7,628,606 teaches a high-velocity oxidizer-fuel (HVOF) torch that employs a stratified stream of oxidizer and fuel reactants to stabilize combustion. The oxidizer is introduced in a vortex constrained in the bore of the torch, and the fuel is introduced axially into a low pressure eye of the oxidizer. The unreacted oxidizer surrounding the combustion gases shields the bore from damage if the bore length is maintained sufficiently short. When a longer bore length is desired, a coolant jacket can be provided around the bore.
It has been observed that for torches of the '606 patent, the hottest region of the exiting flame jet is in a ring due to the outwardly-expanding flame as the reactants pass through the bore of the torch. This limits the effectiveness of the torches in melting high-temperature coating materials that are introduced along the axis of the bore as a powder entrained in the fuel stream.